Method of continuously casting steel

ABSTRACT

The quantity of the continuous casting line equipment necessary to handle a large capacity furnace charge of steel is greatly reduced by dividing the furnace charge into a number of ladles which are successively poured through the casting line equipment while holding the ladles awaiting pouring for times greatly exceeding normal holding times. In this way, processing of the steel may be effected in the ladles, thus reducing the furnace charging time.

United States Patent Olsson 51 Mar. 28, 1972 [54] METHOD OF CONTINUOUSLY CASTING STEEL [72] Inventor: Erik Allan Olsson, Rothfluhstr. 15,8702

Zollikon, Switzerland 221 Filed: May 1, 1910 211 App1.No.: 33,959

' Related U.S. Application Data [63] Continuation-impart of Ser. No. 630,141, Apr. 11, 1967, abandoned, Continuation of Ser. No. 370,067, May 25, 1964, abandoned.

[30] Foreign Application Priority Data May 30, 1963 [52] U.S. Cl. ..164/82, 164/66, 75/46 [51] ..B22d 11/10 [58] Field of Search ..164/52, 56, 66, 82, 259, 281; 266/34 A, 34 PP, 34 T; 75/46, 49, 59

[56] References Cited UNITED s'rnras PA'lElfl'l'S 980,369 1/1911 Walker Sweden 6014/63 2,732,601 1/1956 Junghans ..164/82 2,871,008 H1959 Spire ..75/60 UX FOREIGN PATENTS OR APPLICATIONS 599,137 5/1960 Canada ..164/56 1,223,358 2/1960 France.. ..164/66 760,561 10/1956 Great Britain ..l64/56 OTHER PUBLICATIONS The iron Age, Aug. 19, 1948, T520018. pp. 71, 72 8t 74.

Primary Examiner-R. Spencer Annear Attorney-imirie & Smiley [57] ABSTRACT 4 Claims, No Drawings METHOD OF CONTINUOUSLY CASTING STEEL CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application, Ser. No. 630,141, filed Apr. 1 l, 1967, Streamlined Continuation of Ser. No. 370,067, filed May 25, 1964 both now abandoned.

BACKGROUND OF THE INVENTION The continuous casting process for steel production, while proposed in basic theory many years ago has become successful only within the recent past. The casting machines for producing continuously cast billets and pipe are of relatively small capacity whereas continuous casting machines for producing slabs are of considerably greater capacity. For example, a continuous casting machine for producing steel billets ordinarily provides a capacity of 15 tons/hr. per strand. A plurality of single strand machines may be employed to increase the capacity, or special machines employing multiple strands fed by a common tundish may be used but, in any case, the cost of equipment is directly dependent upon the total number of strands capable of being cast simultaneously.

ln conventional practice, the total capacity of the casting machine equipment is adjusted to be capable of handling the entire furnace charge or batch of steel within some finite time period and for this reason, the casting machine capacity is directly related to and is proportional to the furnace capacity. The reason for this is that the steel must be within rather well defined temperature limits while being cast, which requires that the casting time be within that period in which the temperature of the molten metal remains within these limits. The upper temperature limit is dictated by such temperature as will not cause break-out below the mold, while the lower limit is dictated by the avoidance of the metal freezing prematurely, particularly in the ladle and tundish nozzles.

The temperature range between these limits is in the order of 60 C. and the time period within which such a temperature drop will occur depends on various factors such as the ladle size and amount of molten metal in the ladle, ladle lining performance, effectiveness of heat insulation, and the degree of preheating to which the ladle refractory is subjected. The rate of temperature drop is greatest when the ladle is filled and decreases with time, and is also inversely proportional to ladle size. Then, a usual time for the 60 C. temperature drop in a 5 ton ladle is less than 30 minutes, 45-50 minutes is the usual time for a ton ladle, 60-70 minutes is the usual time for a 45 ton ladle, while 80-100 minutes is the usual time for a 90 ton ladle. By very careful and efficient preheating of the ladle lining, these times may be reduced by as much as 50 percent for the smaller ladle sizes, but with increasing ladle size this technique has less effect.

As the time for the temperature loss of approximately 60 C. is equal to the possible casting time, while the capacity of one billet strand is given by size, form of section, analysis of steel, etc., it is obvious that the number of strands has to be chosen so that the entire heat can be cast within this time. 15 tons per hour and strand is a good average for smaller billet sizes. It would not be possible to cast a whole 15 ton heat within the 45-50 minute time indicated above, but by extremely good preheating of the ladle lining it is possible. Heating of the ladle lining may be achieved by highly efficient oil or gas burners. In addition to this, the steel is sometimes overheated in the furnace permitting a certain holding time in the ladle before starting the casting whereby the superheated steel gives away heat to the ladle lining. This method, however, is hazardous and the use of two strands would be preferred, whereby the ladle is emptied in approximately 30 minutes giving reasonable safety for failures in temperature measurements or in operation ofthe machine.

In the case of 45 ton heats with the above indicated acceptable time of 70-80 minutes in a 3-strand machine would be able to case the entire heat in time and with reasonable degree of safety. (Z-strands would not do the job.)

Casting tons in the above indicated time with a reasonable margin for safety would require at least 5, preferably 6 strands.

When planning continuous casting installation and estimating the number of strands, other factors than those mentioned above have to be carefully considered in order to achieve a reasonable reliability of the operation. The ability at the furnace to supply the steel with sufficient accuracy in temperature to the casting machine is one of those factors. One or two extra strands often are added to those which would normally be required in order to compensate for these factors. Another factor is that the temperature loss from the ladle to the tundish nozzle is dependent on the metal throughflow, which is a function of the size and section being cast. The smaller the throughflow and size, the more reserve has to be considered in the interest of reliability.

In order to increase the reliability with respect to temperature as well as to prolong the casting time and thus niinimize the number of strands it has been proposed to cast from holding furnaces instead of ladles, whereby the entire furnace charge should be supplied to the holding furnace, which would be able to hold the heat until the machine would beprepared to cast and also maintain the temperature during the cast. The holding furnace is therefore placed on top of the casting machine. This method has a very limited practical use, however, because it is expensive to install and operate. The use of electrodes on the top of the machine would be very complicated as the surface levels change very much in bottom pouring type operation and the electrode travel has to be correspondingly long and when a tilting furnace is used the electrodes would come in an unfavorable elevation to the metal surface when tilted. Ordinary induction furnaces are also impractical because their efficiency drops rapidly with the reduced metal content in the furnace, to the extent that at the end of casting, the steel will be cooled down by the coil cooling water rather than heated. A possible way to use induction heating is the channel type furnace, which however has a low effect per channel and a relatively high refractory cost and is therefore suitable only in a few cases. Furthermore, it is very difficult to make such a furnace mobile because of its weight and the necessity to watercool the coils. A ladle for transporting the steel from melting-furnace to holding-furnace is therefore necessary with additional heat loss as a result. Therefore, in most cases, one prefers to utilize the casting equipment with so many strands that the whole heat can be cast in acceptable time, which is generally less than 1 1/2 hours, in most cases, however, not more than about l hour.

According to the present invention, the original investment in casting line equipment is greatly reduced while its per centage of utilization time with respect to the tap to tap time of the furnace or furnaces is greatly increased through the expedient of greatly extending the time during which a furnace charge can be cast. Moreover, the invention brings about an improved accuracy in casting temperature after the steel has been tapped into the ladle and thus an improved reliability. This is accomplished by dividing the furnace charge into a plurality of ladles and then casting the contents of these ladles consecutively through the common casting line equipment. The ladles being held back and awaiting their turn for being cast or subjected to a re-heating step just prior to casting and after their holding times have exceeded about two hours, they are subjected to argon gas treatment in order to prevent unacceptable alteration of the steel analysis which will later occur and in order to stir the metal for equalization. By the possibility of adjusting the metal temperature and the analysis in the ladles, the melting unit may be relieved of some of the steel processing steps and these steps may be performed, instead, in the ladles themselves. In this way, the tap to tap time of the furnace is reduced since the furnace may now operate essentially as a melting device only.

DETAILED DESCRIPTION OF THE INVENTION l have found that relatively large charges of molten steel from a furnace may be held for a considerable period of time without deleterious change in analysis of the steel, provided that the steel is protected from the influence of the atmosphere. Specifically, l have found that a ladle having a capacity of at least about 30 tons may be held for more than one hour without unacceptable change of analysis provided the molten steel is protected by a suitable slag layer. After a certain time, however, the reoxidation of the steel will approach an undesirable degree, but I have found that if, before the time, a relatively small amount of inert gas is allowed to bubble upwardly through the molten steel, deleterious changes in the steel analysis may be avoided indefinitely. For example, a 60 ton ladle protected by slag may be protected from further change in analysis after several hours by introducing argon gas, through a lance inserted to the bottom of the ladle, at a flow rate of only one liter per minute. By such small amounts of gas any noticeable degassing effect cannot be expected, but the gas appears to form a protective blanket between the molten metal and the slag.

in this way, the ladle may be held indefinitely, provided that when the heat losses drop the temperature to a point in which skulls are apt to form, the ladle is reheated to bring it to a temperature of from 20-70 C. above the solidification point. The reheating of the ladle is performed by an electric arc process.

In conventional practice, assuming a furnace charge of 90 tons delivered as a batch every ten hours, and assuming a capacity of 15 tons per strand and hour, six billet strands would be required to handle the furnace charge within the pouring time of l hour. The steel from the 90 ton ladle is distributed to the six molds by a tundishlocated between the ladle and the molds. In some instances it may be more suitable to split the heat between two ladles each being poured in a 3- strand machine. In case only one furnace is available, the 6- strand machine or the two 3-strandmachines would be utilized only 1 hour of 10, as the entire furnace batch or charge has to be poured in 1 hour, which we here assume to be the time for the temperature drop which can be permitted in this special case.

According to the present invention, 6-strancls should not be built, but only three or two. If a 3-strand machine is chosen, the 90 ton furnace heat would be split into two 45 ton ladles, one of which could be taken to the machine after adjustment of the temperature and analysis and cast, which would take approximately I hour. In the meantime, the second ladle has to wait. After approximately 45 minutes waiting time, argon gas is blown in through the bottom of the ladle for protection during the terminal portion of the casting time of the first ladle even though the waiting time of the second ladle is only about one hour. The temperature is checked and the ladle content is heated up to desired casting temperature. Since there are only two ladies involved, the first ladle, when emptied, can now be substituted by the second one which will be cast consecutively for the next hour. By this method the utilization time has been doubled for the casting installation, the cost of which is only half the cost for a 6-strand machine and which may be operated by only half the number of personnel as in case of casting according to conventional practice.

If only two strands are chosen the 90 ton furnace heat is preferably split into three 30 ton ladles. Still assuming a capacity of 15 tons per strand, it will now take 3 hours for the machine to cast the 90 tons. One of the filled ladles can be taken to the casting machine immediately after temperature and analysis corrections. The other two are set aside whereby eventually argon gas is bubbled into the liquid steel, which furthermore is thoroughly covered with slag and powder. Approximately l minutes before the first ladle has been emptied on the casting machine, the second ladle is being prepared for the casting, i.e., is checked on analysis and temperature and given additions and increase of temperature accordingly. The

change of ladles on the machines takes place as soon as the first ladle has been emptied so that the second ladle can be cast without interruption of the casting procedure, ie. the tundish should not run empty. A quick ladle change can be facilitated by using ladle cars or a swingable ladle support on top of the casting machine. The third ladle, the waiting time of which is approximately double that of the second ladle and which has thus had more temperature loss, is taken to the heating station earlier prior to being transported to the casting machine inasmuch as it will take longer to reheat the third ladle content to the desired temperature. The steel temperature at the start of casting the third ladle can, however, be somewhat lower than for the second ladle, and the second ladle in turn may have a little lower starting temperature than the first one inasmuch as the linings of the ladles which have absorbed correspondingly more heat from the steel so that the temperature drops during casting will be progressively less for the second and third ladles. It is therefore conceivable but not practical in view of the operation to use different ladle sizes, the bigger ones being cast after the smaller ones. By using a two strand continuous casting machine and three ladles being cast consecutively, the utilization of the casting unit has been increased from 1 hour of 10 to 3 hours of 10 available (equal tap to tap time) in the same time as the installation cost and the number of operators decreased to approximately one third of that of conventional operation.

It is obvious that the casting installation as proposed would be able to serve more than one furnace. Depending on the possible tap to tap time of the furnaces, the installation could serve 2 to 4 furnaces.

it is also obvious that, as the temperature of the ladle content is increased in a short time, only one heating installation is necessary in contrary to former proposals for holding the heat whereby one heating equipment is necessary for each ladle.

The above given indications on casting capacity per strand and tap to tap time are to be taken only as exemplary, inasmuch as casting capacity varies depending on the size and section of the casting, steel grade and type of casting machine as well as on variations in tap to tap times for different types of furnaces, for different ways of charging, methods of melting and refining the charge, etc.

E.g., a large capacity open hearth of 240 tons having a tap to tap time of 10-12 hours may have its heat cycle reduced considerably by adopting suitable devices for quick charging and using oxygen at melting and refining. If in addition the open hearth is operated substantially only as a melting device as is offered by utilization of the idea of this invention, namely to make the adjustments for the casting in the ladles, the efficiency of the entire operation may be further increased. That is to say, the processing of the steel is not completed in the furnace, but the steel is only provided in its initial stages of processing and in a molten form from the open hearth or furnace. The charge is then divided off into the plurality of ladles required and the processing of the molten steel itself is finished while all of the ladies are being held, still leaving suffcient time for the casting operation.

Thus, a typical plant, according to this invention, consists of one or more furnaces, a heating station, and several holding stations, some of which are provided with argon degassing equipment. The steel from the furnace may be completely or only partially processed. If completely processed, the charge may be divided among several ladies and one of them may be immediately poured while the rest are provided with a protective slag and placed at the various holding stations. Of course, the conventional ladle additives to the steel may also be made, the furnace charge in this event being still considered as completely processed. Near the end of the casting of the first ladle, the second ladle is subjected to heating at the heating station and then poured after the first ladle is cast. Since the first ladle will not require more than 1 1/2 hours of casting time, the second ladle need not be subjected to argon degassing. Third and subsequent ladles will, however, require the use of argon degassing since by oxidation during the relatively long waiting time an out-ot limit change of analysis may occur.

All ladles are heated just prior to casting and, when required, are subjected to an interim heating as described to prevent the formation of skulls.

It is preferred, however, that the steel be processed only partially in the furnace so that the furnace, in effect, is used essentially only as a melting device. As before, the furnace charge is divided among the several ladles and the processing of the steel is completed in the ladles, one at a time, at the heating station. All but the first ladle are provided with a protective slag for holding and argon degassing is effected when required.

As the temperature loss in a big ladle, e.g., 240 tons, is much smaller, casting times of over 2 hours are conceivable, which means that one billet strand could cast approximately 30 tons in 2 hours. A machine with at least 8 strands would then be necessary for emptying the ladle within 2 hours.

This is an extreme but typical example for the benefit of our proposed method:

If the melting shop has only one 240 ton SM-furnace with a tap to tap time of approximately 7 hours, the utilization of the 8-strand machine would be only 28.5 percent. If, according to the present invention, the heat is divided into 3 ladles each of 80 tons which would successively be cast in approximately 80 minutes each, a 4-strand machine would do the job and would yield a utilization of about 57 percent. The machine cost is herewith reduced to approximately half of that of an 8-strand machine and the crew, operating the machine, is also reduced to half. However, the time difference between the casting time and the tap to tap time could now be reduced by removing some of the refining and finishing time of the charge from the furnace to the ladles, thereby increasing the utilization and productivity of the furnace as well as of the machine. The heat could be refined to approximately C-content which, later on when the analysis of the steel in the ladles has been taken, could be adjusted in the ladies together with other additions, e.g., iron-silicon and iron-manganese. As protection, a sufficient slag from the furnace should be charged to the ladles, on top of which an insulating powder, e.g., vermiculite eventually mixed with exothermic material (in Japan, usually rice-raw). This extra insulating precaution may not ever be necessary for the first ladle to be cast.

A quick and efficient method of correcting the carbon content in case it has been too low is to blow pulverized graphite or coke or similar material into the liquid steel below the slag. The part of the addition which does not immediately get absorbed into the steel floats up and stays under the slag layer, thus being protected from a direct reaction with the air but staying in intimate contact with the components, with which it is intended to react.

in order to save set-up time for the machine and tundishes, it is advantageous to adjust the temperature in such a time that the 3 ladles can be cast consecutively without intermediate stopping (back to back casting). When one ladle is close to being empty the following ladle should have been prepared and transported so close to the machine that a ladle change can be made without the tundish running empty.

By using good ladle lining and the newly developed ladle valves instead of stopperrods, which soften at longer holding time, it is conceivable to maintain such a high average temperature of the refractory lining and thus minimizing the temperature loss that the mentioned 240 ton heat could be split into 4 ladles of approximately 60 tons, which could be cast consecutively in a 3-strand machine. Each ladle would then be emptied in about minutes making a total casting time at back to back casting of 5 hours, 20 minutes, i.e., with a utilization of approximately 76 percent.

in any event, when utilizing large capacity furnaces, the total casting time may be made to approach the time required to produce the batch of steel. For example, using two ton furnaces on a staggered basis delivering 90 tons of steel every 5 hours, the casting e%uipment must have a capacity somewhat greater than 9 tons/5 hours 16 tons/hour in order to complete the casting within the available time. Since some time is required to prepare the casting equipment for each successive casting operation, the total utilization time of the casting equipment should not be calculated to exceed about 80 percent of the actual time available if the steel from the furnace is full processed (less usual ladle additives) or about 60 percent of the available time if the steel is to be processed in the ladle. Thus, in the specific example under consideration, the casting equipment should be capable of handling about 22.5 tons per hour if the furnace charge is fully processed and about 30 tons per hour if the steel is to be processed in the ladles. Bearing in mind that the ladle capacity should not be less than about 30 tons, the number of ladles employed in the first case (furnace-processed steel) would be three in order not to exceed the pouring time per ladle of 1 1/2- hours, whereas for the second case, either two 45 ton ladles or three 30 tone ladles could be used.

I claim:

1. The method of continuously casting steel so that the utilization time of the continuous casting equipment approaches the period of time between consecutive heats, which comprises the steps of:

a. tapping a fixed quantity of molten steel every N hours,

b. dividing the entire quantity of the tapped molten steel into a plurality of ladles, each ladle being of capacity such that during a ladle casting time of x hours the temperature of the molten steel in the ladle will drop less than 60 C.,

c. forming a protective slag layer on at least all but a first ladle,

d. sequentially casting the contents of all the ladles, starting with the first ladle, through continuous casting equipment of capacity such that the casting time of each ladle is said x hours,

e. heating the contents of each consecutive ladle awaiting next sequential casting during the terminal portion of the casting period of the'preceding ladle, and

f. introducing an inert gas into the steel of at least the last ladle to be cast and the steel of any ladle whose waiting time between steps (b) and its time of casting exceeds about 2 hours, the inert gas being introduced at a rate sufficient to form a protective blanket over the molten steel beneath the slag layer.

2. The method according to claim 1 wherein the tapped molten steel is fully processed.

3. The method according to claim 1 wherein the tapped molten steel is incompletely processed and including the steps of;

forming a slag layer on the first ladle contents during step heating and finally processing the contents of the first ladle prior to step (c), and,

finally processing the contents of each consecutive ladle during the heating as set forth in step (e).

4. The method according to claim 3 wherein the casting time x for each ladle is in the order of l /22 hours. 

1. The method of continuously casting steel so that the utilization time of the continuous casting equipment approaches the period of time between consecutive heats, which comprises the steps of: a. tapping a fixed quantity of molten steel every N hours, b. dividing the entire quantity of the tapped molten steel into a plurality of ladles, each ladle being of capacity such that during a ladle casting time of x hours the temperature of the molten steel in the ladle will drop less than 60* C., c. forming a protective slag layer on at least all but a first ladle, d. sequentially casting the contents of all the ladles, starting with the first ladle, through continuous casting equipment of capacity such that the casting time of each ladle is said x hours, e. heating the contents of each consecutive ladle awaiting next sequential casting during the terminal portion of the casting period of the preceding ladle, and f. introducing an inert gas into the steel of at least the last ladle to be cast and the steel of any ladle whose waiting time between steps (b) and its time of casting exceeds about 2 hours, the inert gas being introduced at a rate sufficient to form a protective blanket over the molten steel beneath the slag layer.
 2. The method according to claim 1 wherein the tapped molten steel is fully processed.
 3. The method according to claim 1 wherein the tapped molten steel is incompletely processed and including the steps of; forming a slag layer on the first ladle contents during step (c), heating and finally processing the contents of the first ladle prior to step (c), and, finally processing the contents of each consecutive ladle during the heating as set forth in step (e).
 4. The method according to claim 3 wherein the casting time x for each ladle is in the order of 1 1/2 -2 hours. 